There is an ever-increasing demand for more sophisticated, smaller, and less expensive consumer electronic devices in today's high-tech marketplace. Consumer products, such as the optical disc reader and writer (e.g., compact disc (CD), digital versatile disc (DVD), and blu-ray disc (BD) players and writers), are selling in record numbers. As a result, new and innovative technologies for these optical disc readers and writers keep emerging from developers. One area of design in the optical disc readers and writers is the optical disc pick up.
The optical pickup uses DOEs and lasers to optically read or write to an optical disc (e.g., CD, DVD, or BD). Unlike conventional optical components that utilize refraction and/or reflection, the DOE enables parallel processing by optically diffracting and directly controlling the optical phases. Therefore, a wide range of applications, including, for example, multi-spot beam splitters or shapers, can be expected as a result of this preferred benefit. Conventionally, the DOEs in optical pickups are manufactured to be binary, i.e., the DOEs only have two phase levels. Binary DOEs are easier to manufacture because they are compatible with the standard semiconductor fabrication processes, which are well developed. However, binary DOEs generally suffer lower diffraction efficiencies. Additionally, binary DOEs produces symmetric diffraction orders (e.g., a −1 order has the same intensity as a +1 order), whereas for many DOE applications—optical pickups included—asymmetric diffraction orders are desired. These drawbacks result in an inefficient optical pickup. Therefore, there exists a need to provide a more efficient DOE which does not employ binary beam splitting for optical pickups.
The grayscale DOE, a DOE that has more that two phase levels, is a type of DOE that can be more efficient and produce asymmetric diffraction orders. However, grayscale DOEs have been difficult and expensive to fabricate and, therefore, inhibit manufacturers from using them in optical pickups. Therefore, there exists a need to provide a means of inexpensively manufacturing grayscale DOEs.
Currently, there are three major methods of manufacturing grayscale masks which are used to fabricate grayscale DOEs.
First, a method of creating grayscale masks for grayscale DOE fabrication is described in U.S. Pat. No. 5,310,623, entitled, “Method for fabricating microlenses.” The '623 patent details a half-tone method of manufacturing grayscale masks. However, half-tone manufacturing techniques employ a series of binary pixels, which collectively vary the transmission of light approximating a grayscale mask which may be used to create grayscale DOEs. Because this is another binary approach, the resolution is limited. Therefore, there exists a need to provide a means of fabricating grayscale masks with greater resolution than that of half-tone grayscale masks.
Second, a method of creating grayscale masks for grayscale DOE fabrication is described in U.S. Pat. No. 5,078,771, entitled, “Method of making high energy beam sensitive glasses.” The '771 patent details a method whereby a high-energy beam sensitive-glass (HEBS-glass) illuminates the mask with varying intensities, and thereby creates a grayscale mask. However, the HEBS-glass transmission often changes during the exposure times, which results in non-identical grayscale masks. Therefore, there exists a need to provide a means of identically manufacturing a number of grayscale masks.
Third, a method of making a grayscale mask for manufacturing grayscale DOEs is described in U.S. Pat. No. 6,638,667, entitled, “Fabricating optical elements using a photoresist formed using of a gray level mask.” The '667 patent details a method whereby grayscale patterns are created by varying the thickness of a light absorber layer. The varying thickness in the absorber layer is created by using a series of binary masks. However, employing a series of binary masks is a cumbersome and costly means of manufacturing a grayscale mask. Also, a series of binary masks create only an approximation of a true grayscale mask. Therefore, there exists a need to provide a means of creating true grayscale masks in an efficient manner without employing binary masks.
The '667 patent further discloses use of a nickel alloy called “inconel” as the absorber layer. However, because metals have very high light attenuation, the metal layer must be very thin (˜0.1 μm or less) to allow adequate light to transmit through the layer. In practice it is difficult to control the thickness of such layers due to the small overall thickness required—any small variation in thickness can have a large variation in transmitted light. In U.S. Pat. No. 6,613,498, assigned to Mems Optical, the use of SiO for the absorber layer is described. However, an absorber material, such as SiO, has fixed absorption coefficient at a given wavelength. It is difficult to obtain desired total absorption and the desired thickness since the absorption coefficient is fixed—to obtain a desired maximum absorption one must use a fixed thickness. Therefore, there exists a need to provide a means of absorber layer with adjustable levels of absorption. Also, applying the metal layer or SiO absorber by evaporation can be a difficult process. Therefore, there exists a need to provide a means of applying the absorber layer in an easy and convenient way.
It is therefore an object of the invention to provide a means of creating a more efficient DOE for optical pickups which do not employ binary beam splitting.
It is another object of the invention to provide a means of inexpensively manufacturing grayscale DOEs.
It is yet another object of the invention to provide a means of creating true grayscale masks in an efficient manner without employing binary masks.
It is yet another object of the invention to provide a means of fabricating grayscale masks with greater resolution than that of half-tone grayscale masks.
It is yet another object of the invention to provide a means of identically manufacturing a number of grayscale masks.
It is yet another object of the invention to provide a means of easily creating a light-absorbing layer for making a grayscale mask whose absorption and thickness are adjustable to satisfy a broad range of specifications.